Description:

"4.01 Comprehensive Overview G. C. Beroza, Stanford University, Stanford, CA, USA H. Kanamori, California Institute of Technology, Pasadena, CA, USA 2007 Elsevier B.V. All rights reserved. 4.01.1 Introduction 4.01.2 Seismicity 4.01.2.1 Earthquake Size 4.01.2.2 Earthquakes in the Context of Plate Tectonics 4.01.2.2.1 Transcurrent plate boundaries 4.01.2.2.2 Divergent plate boundaries 4.01.2.2.3 Convergent plate boundaries 4.01.2.2.4 Intraplate earthquakes 4.01.2.2.5 Hot spot volcanism 4.01.2.3 The Largest and Deadliest Earthquakes 4.01.2.4 Historic and Prehistoric Earthquakes 4.01.2.5 Earthquake Size Distribution 4.01.2.6 Earthquake Location 4.01.3 The Earthquake Source 4.01.3.1 Point-Source Parameters 4.01.3.2 Sense of Faulting from First Motions 4.01.3.3 Moment-Tensor Representation 4.01.3.4 Seismic Energy 4.01.3.5 Extended-Source Models of Earthquakes 4.01.3.5.1 Kinematic source models 4.01.3.5.2 Dynamic source models 4.01.3.6 Volcano Seismology 4.01.4 Slip Behavior 4.01.4.1 Brittle Failure 4.01.4.2 Creep 4.01.4.3 Aseismic Transients and Tremor 4.01.5 Physics of the Earthquake Source 4.01.5.1 Friction 4.01.5.2 Energy Budget 4.01.5.3 Microscopic Processes 4.01.5.3.1 Hydrological processes 4.01.5.3.2 Melting 4.01.5.3.3 Thermal pressurization 4.01.5.4 Fault-Zone Structure 4.01.5.5 Borehole Observations, Fault-Zone Drilling, and Seismicity in Mines 4.01.5.6 Earthquakes as a Complex System 4.01.6 State of Stress, Nucleation, and Triggering 4.01.6.1 State of Stress 4.01.6.2 Earthquake Nucleation and Short-Term Earthquake Prediction 4.01.6.3 Earthquake Forecasting 4.01.6.4 Static Stress Triggering 4.01.6.5 Dynamic Triggering 4.01.6.6 Temporal Distribution of Earthquakes 4.01.7 Associated Problems 4.01.7.1 Strong Motion Prediction 2 Comprehensive Overview 4.01.8 Tsunamis 4.01.9 Test-Ban Treaty Verification 4.01.10 Solid Earth-Atmospheric Coupling 4.01.11 Earthquake Risk Mitigation 4.01.12 Conclusions References 53 4.01.1 Introduction In general usage, the term earthquake describes a sudden shaking of the ground. Earth scientists, however, typically use the word earthquake somewhat differently - to describe the source of seismic waves, which is nearly always sudden shear slip on a fault within the Earth (see Figure 1). In this article, we follow the scientific usage of the term and focus our review on how earthquakes are studied using the motion of the ground remote from the earthquake source itself, that is, by interpreting the same shaking that most people consider to be the earthquake. The field defined by the use of seismic waves to understand earthquakes is known as earthquake seismology. The nature of the earthquakes makes them intrinsically difficult to study. Different aspects of the Figure 1 Earthquakes are due to slip on faults within the Earth. In large earthquakes, fault slip can reach the Earth's surface. Photo shows that surface rupture of the 1906 San Francisco earthquake offset this fence horizontally by 8.5 ft. Plate 1-B, US Geological Survey Bulletin 324 - from US Geological Survey Photographic Library, San Francisco Earthquake, plate 41. earthquake process span a tremendous range in length scales - all the way from the size of individual mineral grains to the size of the largest plates. They span a tremendous range in timescales as well. The smallest micro-earthquakes rupture faults for only a small fraction of a second and the duration of even the very largest earthquakes can be measured in hundreds of seconds. Compare this with the length of strain accumulation in the earthquake cycle, which can be measured in decades, centuries, and even millennia in regions of slow strain rate. The evolution of fault systems spans longer times still, since that can require the action of thousands of earthquakes. At different physical dimensions or temporal scales, different physical mechanisms may become important, or perhaps negligible. Earthquakes occur in geologically, and hence physically, complicated environments. The behavior of earthquakes has been held up as a type example of a complex natural system. The sudden transformation of faults from being locked, or perhaps slipping quasi-statically, to slipping unstably at large slip speeds, as is nearly universally observed for earthquakes, also makes them a challenging physical system to understand. Despite these challenges, seismologists have made tremendous progress in understanding many aspects of earthquakes - elucidating their mechanisms based on the radiated seismic wavefield, determining where they occur and the deep structure of faults with great precision, documenting the frequency and the regularity (or irregularity) with which they occur (and recur) over the long-term, gaining insight into the ways in which they interact with one another, and so on. Yet, the obvious goal of short-term prediction of earthquakes - that is specifying the time, location, and size of future significant earthquakes on a timescale shorter than decades - remains elusive. Earthquakes are different in this sense from nearly all other deadly natural hazards such as hurricanes, floods, and tornadoes, and even volcanic eruptions, which to varying degrees are predictable over a timescale of hours to days. The worst earthquakes rank at the very top of known disasters. The deadliest known earthquake killed over half a million people in a matter of minutes. A level of sudden destruction that no other catastrophe in recorded history - either natural or human-made - has attained. Our inability to predict earthquakes is one reason they cause such apprehension. This lack of a precursory warning is compounded by the fact that they strike so abruptly. No one can see an earthquake coming and it is only a matter of seconds from the initial perception of the first arriving waves of a large earthquake before dangerous strong ground motion begins. Moreover, large, damaging earthquakes occur infrequently (fortunately) at any given point on the Earth relative to a human lifespan. This means that most people who experience a major earthquake are doing so for the first time. Finally, there is something fundamentally unsettling about the movement of the solid earth. The unpredictability, sudden onset, and their unfamiliarity make earthquakes a uniquely terrifying phenomenon. As testament to this, other extreme and catastrophic events in the affairs of humankind - if they are devastating enough - are described with the simile, like an earthquake. The point of origin of an extreme event of any kind is often described as the epicenter, a term borrowed from seismology. The unpredictability of earthquakes also renders them difficult to study. Since we do not know where and particularly when large earthquakes will strike, collecting data on earthquakes has to be approached passively. Seismologists deploy instruments to measure seismic waves where we expect earthquakes to occur and then wait for nature to carry out the experiment. The wait can last decades or more for a large earthquake. Inevitably, with finite budgets and finite patience, this leads to seismic monitoring instruments being widely, and hence too thinly, dispersed in an attempt to gather data from at least some earthquakes, wherever and whenever they might occur. Finally, the combination of unpredictability, sudden onset, long intervals between events, and unfamiliarity means that the risk created by earthquake hazards is extremely difficult for both policymakers and the general public to contend with. Because earthquakes are not predicted and occur infrequently relative to other hazards, it is understandably tempting for governments and individuals to focus on the many immediate and predictable problems that impact society more frequently. The unpredictability and sudden onset of earthquakes, however, mean that once an earthquake begins, it is too late to do much more than duck and cover." Ключевые слова: e, r, o